And I'm Linda Wertheimer. Today the State Department advised all U.S. citizens to leave Yemen because of extremely high danger for Americans there. The U.S. Air Force has flown some State Department personnel out of the capital, Sana'a. In all, 19 U.S. embassies and consulates remain closed across the Middle East and Africa because of a security threat. We're hearing more about what is causing all the concern. Intelligence officials say they intercepted an electronic communication between two top al Qaida leaders and they were talking about an attack in the works against Western targets.

NPR's counterterrorism correspondent Dina Temple-Raston has been following this story. And she is here with the latest. Dina, tell us about this intercepted communication.

DINA TEMPLE-RASTON, BYLINE: Well, U.S. officials say it's between Ayman al-Zawahiri, the man who replaced Osama bin Laden, and another man named Nasser al-Wuhayshi, who, until a couple of weeks ago, was the head of al Qaida's affiliate in Yemen. I say until a couple of weeks ago, because officials told us that it appears that Wuhayshi was elevated to al Qaida's number two spot last month. Wuhayshi used to have direct connections to bin Laden. He was his secretary.

And some officials think that there might be a plan to commemorate Wuhayshi's new role with a big attack. And the evacuation order shows how concerned they are about this.

WERTHEIMER: But closing embassies and sparking a worldwide travel alert seems a fairly broad brush. I mean, are U.S. officials reacting to specific threat information or more just an abundance of caution?

TEMPLE-RASTON: Well, they have a pretty good idea where the threat might come from - Yemen - but they don't know the target. Officials said that the intercepts entailed Zawahiri, the head of al Qaida, telling this new deputy to launch an attack but there was no sense of where and when, aside from it being soon. So the U.S. decided to close these diplomatic missions out of an abundance of caution.

And clearly, last year's attack on the U.S. diplomatic embassy in Benghazi in which Ambassador Chris Stevens and three others were killed is clearly affecting their thinking here. And the fact that the direction went to the former head of al Qaida's affiliate in Yemen has focused attention on Yemen. You know, U.S. allies like France and Germany and the U.K., they've closed their missions in Yemen until further notice.

The president of Yemen was in Washington last week. He may have also alerted President Obama to something that was going on. And they've been talking about something called the threat stream. Basically, officials hear one thing that worries them - like an exchange between two leaders - and then they look for evidence in the chatter among operatives that sort of supports that.

Officials told me that there appeared to be movement of operatives, intense chatter among people they were listening to, and you bring all those threads together and that reinforces what they heard in the intercept.

WERTHEIMER: Dina, we are talking about al Qaida again. I mean, we hear on the one hand that al Qaida has been severely weakened in the last few years. Is it resurgent now?

TEMPLE-RASTON: Well, al Qaida's role in Yemen has always been an issue. You may remember just last year there was an infiltration of the group and a double agent made off with a new kind of underwear bomb. What's interesting about this latest alert is that Zawahiri, the leader of al Qaida's core group, is reaching out directly to an affiliate, this al Qaida group in Yemen. And instead of just making a generalized call for an attack, he's reached out to a particular person.

So, you know, is this indicating some sort of management structure change for al Qaida? Are they adapting in a new way? We don't know yet but that's something that intelligence analysts are looking at.

WERTHEIMER: It's hard not to notice that this is all happening at a time when the National Security Agency, the NSA, is facing increasing criticism over its surveillance programs on Capitol Hill. Is all that connected?

TEMPLE-RASTON: Well, the House nearly passed legislation last month that would've clipped the NSA's surveillance programs. And then in a hearing last week the Senate Judiciary Committee grilled officials from the director of National Intelligence and the NSA about those surveillance programs. And officials were back on their heels, saying they were open to whatever changes lawmakers wanted to make as long as they could keep the programs.

Now, all these same members of Congress are being briefed on this threat and they're being told that these intercepts were from high ranking al Qaida officials. So the whole subject has changed. Now they're listening to briefings based on the effectiveness of these programs and telling them how important they are. You know, State Department officials have said they want to get back to business, they want to open these consulates, but it looks like this terror threat could last all month.

So the momentum that was building to change the NSA's surveillance programs, it's going to be all gone.